1. Field the Invention
In one aspect, the invention concerns a device for injecting liquid into at least one chamber pressurized by a gas and forming part of a periodic operation machine such as an internal combustion engine, the device comprising a unit for pressurizing the liquid to be injected formed by a variable volume first cavity delimited by a piston reciprocating inside a cylinder,
which first cavity communicates with low-pressure liquid reserve means through the intermediary of a control unit establishing such communication cyclically in synchronism with the periodic operation of said machine and via a passage that is open at all times, except for the presence of a re-aspiration valve or like unit, with an injector formed by a nozzle and a mobile needle which is a body of revolution,
which nozzle comprises a second cavity connected to said passage and delimited laterally by a circular cylindrical wall and axially by a partially conical wall forming a seat and coaxial with said cylindrical wall and at least one injector orifice discharging into said pressurized chamber,
which needle has a cylindrical part forming a piston and adapted to slide with minimal operating clearance inside said cylindrical wall of the nozzle and a conical part cooperating with said seat,
which needle is adapted to be held when idle in bearing engagement with the seat by return means so as to interrupt communication between the second cavity and the injector orifice(s) and so that its cross-section bearing on the seat, when projected onto a plane perpendicular to the axis of the needle, is less than the transverse cross-section of the cylindrical part of the needle forming the piston, the free surface of the cylindrical part of the needle forming the piston delimiting a variable volume third cavity which communicates with a pressurized accumulator of said liquid and thereby constitutes said return means.
The invention is more particularly, but not exclusively, concerned with devices for injecting liquid fuel for internal combustion engines, said pressurized chamber being then formed by the or each working or combustion chamber of the engine.
2. Description of the Prior Art
An injector device of the type defined above is described in FR-A-2.326.588. In this case the piston of the pressurizing unit is caused to reciprocate by a free piston of which one transverse surface is exposed at all times to atmospheric pressure while its other transverse surface is alternately subjected, by the operation of two valves, to the liquid pressure in the accumulator and to atmospheric pressure. Without modifying the operation of the prior art device, the above piston could also be reciprocated by a rotary cam and spring return means system.
Irrespective of how the piston of the pressurizing unit is caused to move, said needle is alternately lifted off its seat by the pressure of the liquid in said second cavity operating on the differential cross-section of the needle against the action of the pressure of the liquid in the accumulator operating in the third cavity and on the full cross-section of the needle and then returned to its seat by this latter pressure after the pressure of the liquid in the second cavity is reduced. The reciprocating motion of the needle in operation is therefore generated entirely by hydraulic means.
In this prior art device the quantity of fuel injected in each cycle is determined by the length of the return stroke of the free piston during the previous cycle, which is determined by the times at which said valves open and close. However, the conditions under which these valves are operated are not known. It should additionally be noted that the prior art device requires, in addition to a liquid discharge at atmospheric pressure, three liquid sources at different pressures, namely said pressurizing unit (600 bars), a high-pressure liquid source (300 bars) to feed the accumulator and a relatively low-pressure liquid source (10 bars) which has multiple functions.
An object of the invention is to adapt the injector device so that the quantity of liquid dispensed in each cycle can be adjusted in a simple manner. Another object is to simplify the prior art design by using only a single source of pressurized liquid, whilst retaining the benefit of this design resulting from the absence of a reciprocating return spring for the needle and therefore the drawbacks associated with a spring of this kind. A preferable further object of the invention is to enable adjustment during periodic operation of the machine of the injection pressure in the second cavity according to the pressure in the pressurized chamber so as to maintain optimum conditions of injected liquid entry and atomization.